Here is the notes for Class 7 NCERT Geography – Chapter 4: Air

Atmosphere

• Our earth is surrounded by a huge blanket of air called atmosphere.
• It provides us the air we breathe.
• It protects us from the harmful effects of the sun’s rays.
• made the temperature on the earth liveable.

Composition of the atmosphere

• • • Nitrogen- 78%
    • Oxygen- 21%
    • Carbon dioxide- 0.03%
    • Argon- 0.93%
    • All others- 0.04%

Nitrogen

• Nitrogen is the most plentiful gas in the air.
• plants need nitrogen for their survival.

Oxygen

• Oxygen is the second most plentiful gas in the air.
• Humans and animals take oxygen from the air as they breathe.
• Green plants produce oxygen during photosynthesis.

Carbon dioxide

• Carbon dioxide is another important gas.
• Green plants use carbon dioxide to make their food and release oxygen.
• Humans or animals release carbon dioxide.
• The amount of carbon dioxide released by humans or animals seems to be equal to the amount used by the plants which make a perfect balance.
• However, the balance is upset by burning of fuels, such as coal and oil.
• They add billions of tons of carbon dioxide into the atmosphere each year.
• As a result, the increased volume of carbon dioxide is affecting the earth’s weather and climate.

Greenhouse

• Carbon dioxide released in the atmosphere creates a greenhouse effect by trapping the heat radiated from the earth.
• It is therefore called a greenhouse gas and without it the earth would have been too cold to live in.

Global warming

• when the level of greenhouse gases in the atmosphere increases due to factory smoke or car fumes, the heat retained increases the temperature of the earth. This is called global warming.
• Effect of global warming
  • Drastic changes in climate
  • Melting of snow in coldest part
  • Rise of sea level

Structure of Atmosphere

Our atmosphere is divided into five layers starting from the earth’s surface.

• • 1. Troposphere
       • This layer is the most important layer of the atmosphere.
       • Its average height is 13 km.
       • The air we breathe exists here.
       • Almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.
    2. Stratosphere
       • Above the troposphere lies the stratosphere.
       • It extends up to a height of 50 km.
       • This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes.
       • It contains a layer of ozone gas.
    3. Mesosphere
       • This is the third layer of the atmosphere.
       •  It lies above the stratosphere. It extends up to the height of 80 km.
       • Meteorites burn up in this layer on entering from the space
    4. Thermosphere
       • In thermosphere temperature rises very rapidly with increasing height.
       • Ionosphere is a part of this layer.
       • It extends between 80-400 km. This layer helps in radio transmission.
    5.  Exosphere
       • The upper most layer of the atmosphere is known as exosphere.
       • This layer has very thin air.
       • Light gases like helium and hydrogen float into the space from here.

Weather

• Weather is this hour-to-hour, day to day condition of the atmosphere.
• Weather can change dramatically from day to day.

Climate

• the average weather condition of a place for a longer period of time represents the climate of a place.

Temperature

• The degree of hotness and coldness of the air is known as temperature.
• The temperature of the atmosphere changes not only between day and night but also from season to season.
• Insolation: It is the incoming solar energy intercepted by the earth.
  • The amount of insolation decreases from the equator towards the poles.
• Temperature is measured in degree celsius.
  • Invented by Anders Celsius.
  • Scales 0 to 100°C.

Air Pressure

• Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.
• As we go up the layers of atmosphere, the pressure falls rapidly.
• The air pressure is highest at sea level and decreases with height.
• Horizontally the distribution of air pressure is influenced by temperature of air at a given place.
• Low pressure: In areas where temperature is high the air gets heated and rises. This creates a low-pressure area. Low pressure is associated with cloudy skies and wet weather.
• High pressure: In areas having lower temperature, the air is cold. It is therefore heavy. Heavy air sinks and creates a high pressure area. High pressure is associated with clear and sunny skies.
• The air always moves from high pressure areas to low pressure areas.

Wind

• The movement of air from high pressure area to low pressure areas is called wind.
• A wind is named after the direction from which it blows, e.g. the wind blowing from the west is called westerly.

• Three types of wind

1. 1. 1. 1. Permanent winds
            • The trade winds, westerlies and easterlies are the permanent winds.
            • These blow constantly throughout the year in a particular direction.
         2. Seasonal winds
            • These winds change their direction in different seasons. For example monsoons in India.
         3. Local winds
            • These blow only during a particular period of the day or year in a small area. For example, land and sea breeze, loo.

Moisture

• When water evaporates from land and different water bodies, it becomes water vapour.
• When the water vapour rises, it starts cooling.
• The water vapour condenses causing formation of droplets of water.
• Clouds are just masses of such water droplets.
• When these droplets of water become too heavy to float in air, then they come down as precipitation.
• Precipitation that comes down to the earth in liquid form is called rain.

• Three type of rainfall

1. 1. 1. 1. Convectional rainfall
            • Occurs due to heating of the Earth’s surface.
            • Hot air rises, cools at higher altitudes, and condenses to form clouds.
         2. Orographic (relief) rainfall
            • Happens when moist air is forced to rise over a mountain or hill.
            • As the air rises, it cools and condenses to cause rain on the windward side.
            • The leeward side receives little or no rain and is called a rain shadow area.
         3. Cyclonic (Frontal) rainfall
            • Occurs when two air masses of different temperatures meet.
            • The warm air rises over the cold air, cools down, and causes rain.
            • Associated with cyclones and depressions.